Multichannel devices with improved performance and methods of making the same

ABSTRACT

A transistor device is provided that comprises a base structure, and a superlattice structure overlying the base structure and comprising a multichannel ridge having sloping sidewalls. The multichannel ridge comprises a plurality of heterostructures that each form a channel of the multichannel ridge, wherein a parameter of at least one of the heterostructures is varied relative to other heterostructures of the plurality of heterostructures. The transistor device further comprises a three-sided gate contact that wraps around and substantially surrounds the top and sides of the multichannel ridge along at least a portion of its depth.

RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 14/533752, filed 5 Nov. 2014, which is incorporated herein inits entirety.

TECHNICAL FIELD

The present invention relates generally to electronics, and moreparticularly to multichannel devices with improved performance andmethods of making the same.

BACKGROUND

Certain heterostructure materials, such as Aluminum Gallium Nitride(AlGaN) and GaN, create an electron well (i.e., a sheet of electrons) atthe interface between the two dissimilar materials resulting from thepiezoelectric effect and spontaneous polarization effect therebetween.The resulting sheet of electrons that forms at this interface aretypically referred to as a Two-Dimensional Electron Gas (“2DEG”)channel. Equally applicable is a superlattice structure having aplurality of two-dimensional hole gas (2DHG) channels. Both types ofstructures can be referred to as “2D×G channel(s)” devices. FETs thatoperate by generating and controlling the electrons in the 2D×G channelare conventionally referred to as high electron mobility transistors(“HEMTs”).

By stacking a plurality of these two-material heterostructures, and withthe addition of appropriate doping in the layers to maintain thepresence of the 2D×G channels when stacking a plurality ofheterostructure layers, the electron sheets are able to act in parallel,allowing for greater current flow through the superlattice device. Whenthis type of FET is “on”, the superlattice device has a loweron-resistance, relative to a single heterostructure-layer device,because the multiple 2DEG channels allow a proportionally higher currentto flow between the source and drain, resulting in an overall reductionin on-resistance. This type of structure has been well suited forproviding an ultra low channel resistance high frequency switch.

In SLCFETs, multiple pairs of barrier and channel epitaxial layers aregrown (e.g., AlGaN/GaN) to produce multiple conducting two dimensionalelectron gas (2DEG) channels. Due to difficulties in pinching these offfrom the top, the epitaxial is instead etched into ridges and pinchedoff via a gate contact from the sides. Due to limits imposed on bothridge width and sidewall slope by fabrication techniques, these FETstend to show significantly higher pinch-off voltages and electric fieldsthan do most standard, single channel, top pinching FETs. Specifically,for top pinching standard FETs, the gate is very close to thechannel—typically on the order of 5-20 nanometers (nm), thus allowingsmall pinch-off voltages. However, for sidewall gates pinching-offetched ridges, distances from gate to the center of the ridge typicallyare below 100 nm—significantly increasing the necessary pinch-offvoltage.

Though one would ideally want straight ridge sidewalls, typical limitsin fabrication cause these ridges to be etched with sloped sidewallsinstead of straight sidewalls. Such sidewall slopes lead to largerpinch-off voltages being required for the lower channels than for theupper channels because of their wider ridge widths. In the case of veryshallow slopes, such non-uniformity in the pinch-off voltage can causethe lower channels to be too wide to pinch-off since the device willbreak down due to high electric fields at the edge of the sidewall gatesbefore the high pinch-off voltage can be reached. Also, a non-uniformpinch-off voltage per channel means that channels that pinch-off earlyno longer contribute conduction current, thus increasing the on-stateresistance from its optimal.

SUMMARY

In accordance with an example, a transistor device is provided thatcomprises a base structure, and a superlattice structure overlying thebase structure and comprising a multichannel ridge having sidewalls. Themultichannel ridge comprises a plurality of heterostructures that eachform a channel of the multichannel ridge, wherein a parameter of atleast one of the heterostructures is varied relative to otherheterostructures of the plurality of heterostructures. The transistordevice further comprises a gate contact that wraps around andsubstantially surrounds the top and at least one side of themultichannel ridge along at least a portion of its depth.

In accordance with another example, a super-lattice castellated gateheterojunction field effect transistor (SLCFET) is provided. The SLCFETcomprises a base structure, and a superlattice structure overlying thebase structure and comprising a plurality of multichannel ridges havingsidewalls and being spaced apart from each other by non-channelopenings. The multichannel ridge comprises a plurality ofheterostructures that each form a portion of a channel of the SLCFETalong with each other parallel heterostructures of the plurality ofmultichannel ridges, wherein a parameter of at least one of the parallelheterostructures of each of the multichannel ridges is varied. TheSLCFET also comprises a gate contact that wraps around and substantiallysurrounds the top and sides of each the plurality of multichannel ridgesalong at least a portion of its depth and is interconnected togetherthrough the non-channel openings.

In yet another example, a method of forming a transistor device isprovided. The method comprises forming a superlattice structurecomprising a plurality of heterostructures over a base structure bysequentially depositing each layer of a plurality heterostructures overthe base structure with one layer of each heterostructure being doped,and etching away openings in the superlattice structure over a channelregion to form a castellated region in the channel region of alternatingmultichannel ridges with edges and non-channel openings, wherein aparameter of at least one corresponding parallel heterostructures ofeach of the multichannel ridges is varied. The method further comprisesperforming a gate contact fill process to form a gate contact that wrapsaround and substantially surrounds the top and sides of each theplurality of multichannel ridges along at least a portion of its depthand connects each one of the alternating multichannel bridges to oneanother through the non-channel openings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a top plan view of an example of a SLCFET devicestructure.

FIG. 1B illustrates a cross-sectional view of the example of the SLCFETdevice structure of FIG. 1A along the lines A-A.

FIG. 2 illustrates a set of graphs that provide a comparison of a flatand modulated AlGaN barrier doping profiled, and its effect on pinch-offvoltage.

FIG. 3 illustrates multichannel ridge cross-sections showing (a)depletion of multiple channels for constant 2DEG concentration versuschannel number, and (b) decreasing 2DEG concentration versus channelnumber.

FIG. 4 illustrates a single channel ridge cross-section and a set ofdifferent thickness layered multichannel ridge cross-sections and theeffects of fringing electric fields on these structures.

FIG. 5 illustrates a cross-sectional view of another example of a SLCFETdevice structure.

FIG. 6 illustrates a cross-sectional view of yet another example of aSLCFET device structure.

FIG. 7 illustrates a cross-sectional view of an epitaxial structure inits early stages of fabrication.

FIG. 8 illustrates a cross-sectional view of the structure of FIG. 7after undergoing an epitaxial growth process.

FIG. 9 illustrates a cross-sectional view of the structure of FIG. 8after undergoing another epitaxial growth process.

FIG. 10 illustrates a cross-sectional view of the structure of FIG. 9after undergoing yet another epitaxial growth process.

FIG. 11 illustrates a cross-sectional view of the structure of FIG. 10after formation of an etch mask and while undergoing an etch process.

FIG. 12 illustrates a cross-sectional view of the structure of FIG. 11after undergoing the etch process and after the stripping of the etchmask.

FIG. 13 illustrates a cross-sectional view of the structure of FIG. 12after undergoing a gate contact fill process.

DETAILED DESCRIPTION

Multichannel devices with improved performance are disclosed along withmethods of making these multichannel devices. The techniques disclosedherein can provide for optimization of one or more device parameterssuch as, for example, the breakdown voltage, the pinch-off voltage,linearity and other device parameters. The device can be a side-pinchinggate control contact multi-channel device, such as a super-latticecastellated gate heterojunction field effect transistor (SLCFET). TheSLCFET can be used as low-loss switches for a variety of applicationssuch as time delay units, low loss phase shifters and attenuators,switch matrices, T/R switches and circulator replacements or asamplifiers. Though such multi-channel devices offer low on-stateresistance, pinch-off voltages can be very high with such side-pinchingstructures, such that the pinch-off voltages can be high enough tosignificantly limit the device breakdown voltage or prevent the devicefrom pinching-off entirely.

The SLCFET is a solid-state RF switch technology that offers 2-3 timesimprovement in on-state resistance (R_(on)) versus conventionaltechnology through the use of stacked semiconductor channels. However,SLCFET devices have limitations in their breakdown and pinch-off voltagedue to the high electric fields inside the devices. The presenttechniques mitigate the problems associated with these high electricfields, and facilitate a SLCFET that successfully supports highervoltage and power modes of operation.

It is to be appreciated that the sidewall pinch-off voltage for aparticular channel is dependent on the ridge width for that channel.Specifically, V_(po)˜N*W_(ridge) ² where, for that particular channel,V_(po) is the pinch-off voltage, W_(ridge) is the ridge width, and N isthe channel carrier concentration which, for such SLCFETs is the twodimensional electron gas (2DEG) concentration. Though one would ideallywant straight ridge sidewalls, typical limits in fabrication cause theseridges to be etched with sloped sidewalls instead of straight sidewalls.For example, the ridge width of a first channel can be over four timessmaller than that of the sixth channel. Such sidewall slopes leads tolarger pinch-off voltages being required for the lower channels than forthe upper channels because of their wider ridge widths. In the case ofvery shallow slopes, such non-uniformity in the pinch-off voltage cancause the lower channels to be too wide to pinch-off since the devicewill break down due to high electric fields at the edge of the sidewallgates before the high pinch-off voltage can be reached. Also, anon-uniform pinch-off voltage per channel means that channels thatpinch-off early no longer contribute conduction current, thus increasingthe on-state resistance from its optimal.

In one example, a modulated doping technique is provided to optimize thepinch-off voltage of each of multiple 2DEG (two-dimensional electrongas) or 2DHG (two-dimensional hole gas) channels by varying the carrierconcentration of the channels versus channel number within themulti-channel epitaxial stack to provide a substantially equal pinch-offvoltage for each channel. Since such a side pinching device requires theetching of ridges into the multi-channel stack, variations in carrierconcentration versus channel can compensate for a sloped sidewallprofile that is introduced during etching. Tailoring can be adjusted fora fixed pinch-off voltage per channel, minimizing the specificon-resistance of the device for a given pinch-off voltage. Suchmodulated doping can alternately be used to optimize the linearity ofthe FET by tailoring the pinch-off profile for each channel, adding upto a more linear net transconductance. It is to be appreciated that themodulated doping can be employed to optimize other parameters of themultichannel device.

As previously stated, the pinch-off voltage for a particular channeldepends both on its width and on its carrier concentration (2DEGconcentration). Therefore, the 2DEG concentration of the channel can beused as a second variable that can compensate for the widening ridgewidth in a particular channel in order to keep the pinch-off voltageconstant for each channel. Alternatively, it can be used to determine anarbitrary profile for V_(po) versus channel. This is graphicallyillustrated in FIG. 2, which illustrates a first graph 40 of widthversus channel number. A second graph 42 illustrates a dopantconcentration in the barrier layer versus channel number for both auniform dopant concentration shown with solid dots and a modulateddopant concentration shown with solid boxes. It is to be appreciatedthat doping in the barrier layer gives rise to and is roughlyproportional the 2DEG concentration. A third graph 44 compares thepinch-off voltage dependence versus channel for both a flat AlGaNbarrier doping case (i.e. flat 2DEG concentration per channel shown withsolid dots) and one which is scaled as ˜W_(ridge) ⁻² in order to attaina roughly flat pinch-off profile (shown with solid boxes).

There are at least three purposes for which one can tailor the channelconcentrations. The first, as stated, is to keep V_(po) constant perchannel. This minimizes the on-state resistance of the FET for a givenpinch-off voltage. A second method is to tailor the channelconcentrations to achieve a uniform breakdown voltage for each channel,thus minimizing the on-state resistance for a given breakdown voltage. Athird approach is to tailor the channel concentration to achieve a morelinear response of the FET by achieving a more linear transconductanceversus gate voltage response. This is achieved by tailoring the carrierconcentration of each channel to achieve a particular transconductanceresponse for each channel which, when summed up, result in a net FETtransconductance that is more linear. In practice, for GaN SLCFETs, the2DEG concentration for the channels below the top channel is determinedby the doping in the barrier layer (AlGaN). So, it is by grading thedoping concentration of the barrier layers that one can achieve thedesired V_(po) profile.

The general effect of a uniform 2DEG concentration per channel profileon Vpo profile can be seen graphically in FIG. 3(a), which show crosssections of the ridge, with increasingly depleted 2DEG distributions asa function of gate bias for constant 2DEG concentration for each channelregardless of width. The general effect of 2DEG concentration profile onVpo profile can be seen graphically in FIG. 3(b), which show crosssections of the ridge, with increasingly depleted 2DEG concentrationdistributions as a function of gate bias with decreasing dopingconcentration for increasing channel width.

It is also to be appreciated that fringing electric fields caused by theapplied voltage on the gate contact aids in the function of pinching offthe device. However, free carriers through various channels in amultichannel superlattice structure can act as shields to block thesefringing fields. Specifically, for the single channel stack shown inFIG. 4(a), the channel is depleted by both lateral fields from the sidegate and by vertical components of fringing fields that are able toreach that channel. However, for the tightly spaced multi-channel stacksshown in FIG. 4(b), the helpful fringing fields are blocked fromreaching the inner channels due to shielding caused by free carriersthrough the outer (top and bottom) channels. However, if one loosens thespacing between channels by thickening up the channel layers (thickerGaN in this example), then fringing fields can more easily penetrateinto the ridge to aid in the pinch-off of inner channels as well, asillustrated in FIG. 4(c). Furthermore, it has been found from simulationthat most of the pinch-off voltage benefit can be derived simply bygrowing thicker channel epitaxial for alternate channels, as illustratedin FIG. 4(d), as opposed to all channels. Since thicker epitaxial stackgrowth presents its own problems (both in the epitaxial growth and insubsequent fabrication), the alternate thicker layer epitaxial growthcan mitigate these problems associated with the thicker epitaxiallayers.

Therefore, in another example, a channel thickness variation techniqueis disclosed that involves thickening of various channel epitaxiallayers to allow fringing electric fields to aid in the pinch-off of thedevice. These thickened channels can be each inner channel besides thetop and bottom channels, or alternatively, every other inner channelbesides the top and bottom channels. It can be further appreciated thateither of the thickening of the channels techniques can be combined withthe modulated doping technique.

Although these examples methods are illustrated with GaN SLCFETs, theyare equally applicable to any other material system that can producemultiple channels perpendicular to the surface, and are equallyapplicable to any other side-pinching device for such a multi-channelstack.

These methods alone or in combination limit the voltage necessary topinch-off the device while maintaining a low on-state resistance, andalso enable the drain to be driven to higher voltages before breakdown.The modulated doping method can tailor the pinch-off voltages ofindividual channels to produce a fixed pinch-off voltage, which canminimize the on-state resistance for a given pinch-off voltage.Alternatively, non-uniform pinch-off voltages can be tailored for otheruses, for example, to improve linearity. The second method, which usesthicker channel epitaxial method, is for reducing the pinch-off forchannels near this epitaxial.

As an example, a SLCFET structure described herein includes amultichannel device composed of a plurality of aluminum gallium nitride(AlGaN)/gallium nitride (GaN) heterojunction layers of a prescribedthickness and composition, which is known as an AlGaN/GaN superlattice,formed on a single integrated circuit. A 2-dimensional electron gas(2DEG) forms at the interface between the AlGaN and GaN layers resultingin high electron mobility. Equally applicable is a superlatticestructure having a plurality of two-dimensional hole gas (2DHG)channels. Both types of structures can be referred to as “2D×Gchannel(s)” devices.

Example methods of fabrication involve sequential growth of multichannelprofiles in a monolithic epitaxial scheme. By sequentially growing theepitaxial multichannel devices, all devices fabricated from thisstructure will benefit from the inherently high quality materialproperties, atomically flat interfaces and compositional controlassociated with epitaxial growth.

Carriers which form a 2DEG in a standard channel of AlGaN/GaN arespontaneously generated and maintained due to piezoelectric andspontaneous polarization. In this case, the AlGaN barrier is strained byvirtue of its epitaxial relationship with the GaN channel and sincethese materials are piezoelectric, free carriers are generated in thechannel. Since carrier concentration in AlGaN/GaN heterostructures iscontrolled largely by the strain state of barrier and channel layers,the present examples disclose that the SLCFET structure can be createdthrough precise control of composition, thickness and order of AlGaN andGaN layers. An epitaxial scheme and device fabrication method isprovided that exploits this phenomenon which is unique to the GaN-basedmaterial system, and allows for fabrication of low loss RF switches.

Although the present example is illustrated with respect to employing alayer of AlGaN overlying a layer of GaN for each heterostructure, avariety of heterostructures could be employed as long as theheterostructure comprises two layers of dissimilar materials designed tocreate a sheet of electrons (i.e. a 2DEG channel) or a sheet of holes(i.e., a 2DHG channel) at the interface between the two dissimilarmaterials. Various heterostructure materials are known to produce 2DEGand 2DHG channels at the interface therebetween, including but notlimited to Aluminum Gallium Nitride (AlGaN) and Gallium Nitride (GaN),Aluminum Gallium Arsenide (AlGaAs) and Gallium Arsenide (GaAs), IndiumAluminum Nitride (InAlN) and Gallium Nitride (GaN), alloys of Silicon(Si) and Germanium (Ge), and noncentrosymmetric oxidesheterojunctionoverlying a base structure.

It is to be appreciated that in a single heterostructure, the dissimilarmaterials cause an imbalance stress in the system which allowspolarization and/or piezoelectric effects to induce carriers to form ina potential well formed by the heterostructure, i.e., to create a 2D×Gchannel. However, in a superlattice structure where multipleheterostructures are stacked on top of each other, all heterostructuresbeneath the top heterostructure will become balanced and carriers thatwould otherwise induce in those potential wells will not be induced.Adding a doping layer near the interface where the potential well isformed will act to induce the carriers into that potential well,replacing the effect that would have occurred but for the balancingeffect of multiple symmetric heterostructures.

Although the examples herein are illustrated with respect to athree-sided gate, a two-sided gate that covers the top and at least oneside of the gate could be employed. Additionally, a four-sided gatecould be employed that covers the top, bottom and sides of portions ofthe multilayer epitaxial stack. It is further appreciated that adielectric layer can be employed to provide an interface between thegate contact and the multilayer stack. It is yet further appreciatedthat modulating doping for one or more heterostructures of thesuperlattice structure can be employed for a variety of differentreasons that are not illustrated or limited by the present examples. Itis yet further appreciated that varying the thickness for one or moreheterostructures of the superlattice structure can be employed for avariety of different reasons that are not illustrated or limited by thepresent examples.

FIG. 1A illustrates a top plan view of an example of a SLCFET devicestructure 10. FIG. 1B illustrates a cross-sectional view of the SLCFETdevice structure 10 of

FIG. 1A along the lines A-A. The device structure 10 includes acastellated gate contact 25 that substantially surrounds three sides ofa plurality of superlattice structure ridges (multichannel ridges) 20formed from plurality of heterostructures of an AlGaN layer overlying aGaN layer, which are connected to one another through non-channelopenings 27. The castellated gate contact 25 resides in between a draincontact 21 and a source contract 23. Each superlattice structure ridge20 includes a first heterostructure 22 that overlies a base structure12, a second heterostructure 24 that overlies the first heterostructure22 and a third heterostructure 26 that overlies the secondheterostructure 24. The first heterostructure 22 is formed of a firstAlGaN layer 30 overlying a first GaN layer 28. The secondheterostructure 24 is formed of a second AlGaN layer 34 overlying asecond GaN layer 32. The third heterostructure 26 is formed of a thirdAlGaN layer 38 overlying a third GaN layer 36. Each parallelheterostructure, for example, first heterostructures 22 of eachsuperlattice structure 20 form a first channel, second heterostructures24 of each superlattice structure ridge 20 forms a second channel, andthird heterostructures 26 of each superlattice structure ridge 20 form athird channel. Due to imperfect etching techniques, the width of eachlayer decreases from the first GaN layer 28 to the third AlGaN layer 38.This non-uniform width of the heterostructures gives rise to non-uniformpinch-off voltage characteristics in each channel.

The base structure 12 can comprise a substrate layer 14, a nucleationlayer 16 and a buffer layer 18 of, for example, silicon carbide (SiC),aluminum nitride (AlN) and aluminum gallium nitride (AlGaN),respectively. Optionally, the base structure 12 can comprise a substratelayer 14 of sapphire, a nucleation layer 16 comprising a combination ofa low-temperature GaN layer and a high-temperature GaN layer, and abuffer layer 18 based on the material of the superlattice structure tobe located thereon, if required. The base structure 12 can alternativelycomprise a substrate layer 14 of (111)-orientated crystalline silicon, anucleation layer 16 comprising AlN and a buffer layer 18 comprising aseries of discrete AlGaN layers (typically between two and eightlayers), each discrete layer having a different aluminum composition.Other base structures will be apparent to those skilled in the art.

During the deposition, growth or other formation process of each AlGaNlayer, a delta doping of an n-type dopant such as silicon (Si) can beadded at the beginning of each of such AlGaN layers by introducing thedopant gas for a brief time interval, allowing the delta-doping for aconfined thickness within the AlGaN layers to induce a 2DEG channelwithin each heterostructure. Similarly, a p-type dopant such asmagnesium (Mg) could be introduced to induce a 2DHG channel in relevantheterostructures, as applicable. Other n-type and p-type dopants will beapparent to those skilled in the art. In one example, the delta-dopingconcentration is preferably between about 1E17 cm⁻³ and about 1E20 cm⁻³.Other doping profiles will be apparent to those skilled in the art.

In accordance with the example, of FIG. 1B, the doping is modulated orvaried in concentration to provide for substantially equal pinch off foreach channel. For example, the first AlGaN layer 30 of the firstheterostructure 22 has a first dopant concentration, the second AlGaNlayer 34 of the second heterostructure 24 has a second dopantconcentration, and the third AlGaN layer 38 of the third heterostructure26 has a third dopant concentration, wherein the third dopantconcentration is greater than the second dopant concentration and thesecond dopant concentration is greater than the first dopantconcentration. The higher the dopant concentration, the higher thecarrier concentration in the given channel during operation. In thismanner, since the pinchoff voltage depends on both carrier concentrationand the ridge width, an increase in dopant concentration can be employedto compensate for narrow ridge widths, and a decrease in dopantconcentration can be employed to compensate for wider ridge widths tosubstantially equalized pinch-off voltage for each channel.

In the example of FIG. 1B, each GaN layer of the heterostructures canhave a thickness of about 5 nm to about 20 nm (e.g., 10 nm) and eachAlGaN layer can have a thickness of about 5 nm to about 15 nm (e.g., 7nm). The castellated gate contact 25 is formed in a central longitudinalportion of a channel region that extends along at least a portion of thedepth of the plurality of superlattice structure ridges 20 formedbetween castellated trench openings 27 that form non-channel openings.

Although the superlattice structure ridges 20 are each illustrated ashaving three heterostructures, the superlattice structure ridges 20 cancomprise a plurality of heterostructures, between 2 and N, wherein N isdefined as the maximum number of heterostructures that can be grown,deposited or otherwise formed on each other without cracking or othermechanical failure in the layers or 2DEG channels. As will beappreciated by one skilled in the art, the value of N is a function ofthe specific heterostructure materials and thickness of each layer. Thespecific number of heterostructures within the superlattice structureand the heteromaterials can vary, and the relative positions of AlGaNand GaN may be reversed, as will be appreciated by one skilled in theart.

The percentage of aluminum in the AlGaN layers 30, 34 and 38 can rangefrom about 0.1 to 100 percent. For example, the percentage of aluminumin the AlGaN layers can be between about 20% and 100% aluminum-contentaluminum gallium nitride. The AlGaN layers can be graded or non-graded.In one example embodiment, each AlGaN layer within the multi-channelstack is graded, the term “graded” being used to denote the process ofgradually changing the percentage of aluminum to its specifiedpercentage, relative to the percentage of gallium, as known in the art.

As stated above, another method to reduce and/or control the pinch-offvoltage of laterally pinching multi-channel epitaxial stacks is toincrease the thickness of the middle channel epitaxial layers. This hasthe effect of allowing the fringing electric fields to penetrate intothe interior of the ridge, as opposed to only the top and bottomchannels in the example of FIG. 1B, thus aiding in pinch-off andreducing the overall pinch-off voltage.

FIG. 5 illustrates a cross-sectional view of another example of a SLCFETdevice structure 50. The device structure 50 includes a castellated gatecontact (not shown) that substantially surrounds three sides of aplurality of superlattice structure ridges (multichannel ridges) 60formed from plurality of heterostructures of an AlGaN layer overlying aGaN layer. Each superlattice structure ridge 60 includes a firstheterostructure 62 that overlies a base structure 52, a secondheterostructure 64 that overlies the first heterostructure 62, a thirdheterostructure 66 that overlies the second heterostructure 64, and afourth heterostructure 68 that overlies the third heterostructure 66.The first heterostructure 62 is formed of a first AlGaN layer 72overlying a first GaN layer 70. The second heterostructure 64 is formedof a second AlGaN layer 76 overlying a second GaN layer 74. The thirdheterostructure 66 is formed of a third AlGaN layer 80 overlying a thirdGaN layer 78. The fourth heterostructure 68 is formed of a fourth AlGaNlayer 84 overlying a fourth GaN layer 82. As stated above, the basestructure 52 can comprise a base layer 54, a nucleation layer 56 and abuffer layer 58 formed of a variety of different materials as describedabove.

In accordance with the example of FIG. 5, during the deposition, growthor other formation process of each AlGaN layer, the doping is uniformlyapplied to each heterostructure. However the thickness of the middleheterostructures 64 and 66 of the superlattice structure ridges 60 areincreased in thickness to facilitate the effect of allowing the fringingelectric fields to penetrate into the interior of the ridges of themiddle heterostructures 64 and 66, thus aiding in pinch-off and reducingthe overall pinch-off voltage of the middle channels. In particular, thethickness of the second GaN layer 74 of the second heterostructure 64and the thickness of the third GaN layer 78 of the thirdheterostructures 66 are increased relative to the thickness of the firstGaN layer 70 of the first heterostructure 62 and the thickness of thefourth GaN layer 82 of the fourth heterostructure 68.

FIG. 6 illustrates a cross-sectional view of another example of a SLCFETdevice structure 100. The device structure 100 includes a castellatedgate contact (not shown) that substantially surrounds three sides of aplurality of superlattice structure ridges (multichannel ridges) 110formed from plurality of heterostructures of an AlGaN layer overlying aGaN layer. Each superlattice structure ridge 110 includes a firstheterostructure 112 that overlies a base structure 102, a secondheterostructure 114 that overlies the first heterostructure 112, a thirdheterostructure 116 that overlies the second heterostructure 114, afourth heterostructure 118 that overlies the third heterostructure 116,and a fifth heterostructure 120 that overlies the fourth heterostructure118. The first heterostructure 112 is formed of a first AlGaN layer 124overlying a first GaN layer 122. The second heterostructure 114 isformed of a second AlGaN layer 128 overlying a second GaN layer 126. Thethird heterostructure 116 is formed of a third AlGaN layer 132 overlyinga third GaN layer 130. The fourth heterostructure 118 is formed of afourth AlGaN layer 136 overlying a fourth GaN layer 134. The fifthheterostructure 120 is formed of a fifth AlGaN layer 140 overlying afifth GaN layer 138. As stated above, the base structure 102 cancomprise a base layer 104, a nucleation layer 106 and a buffer layer 108formed of a variety of different materials as described above.

In accordance with the example of FIG. 6, during the deposition, growthor other formation process of each AlGaN layer, the doping is uniformlyapplied to each heterostructure. However the thickness of every othermiddle heterostructure 114 and 118 of each of the superlattice structureridges 110 are increased in thickness to facilitate the effect ofallowing the fringing electric fields to penetrate into the interior ofthe ridges, of the middle channels formed from the middleheterostructures 114, 116 and 118, thus aiding in pinch-off and reducingthe overall pinch-off voltage. In particular, the thickness of thesecond GaN layer 126 of the second heterostructure 114 and the thicknessof the fourth GaN layer 134 of the fourth heterostructures 118 areincreased relative to the thickness of the first GaN layer 122 of thefirst heterostructure 112, the thickness of the third GaN layer 130 ofthe third heterostructure 116 and the thickness of the fifth GaN layer138 of the fifth heterostructure 120. It is to be noted that in theevery other thicker GaN layer example, the fringing fields are addedfrom one side for each channel, while the every thicker GaN layerexample, the fringing fields are added on both sides of each channel.

Turning now to FIGS. 7-13, an example fabrication is discussed inconnection with formation of the example of a SLCFET device structurethat utilizes a combination of the techniques discussed in FIGS. 1-6,such that the resultant structure has both a thicker middle layer tofacilitate the utilization of fringing fields, and modulated dopingconcentrations for each channel based on channel width. FIG. 7illustrates a cross-sectional view of an epitaxial structure in itsearly stages of fabrication. The epitaxial structure includes a basestructure 150. As stated above, the base structure 150 can comprise abase layer 152, a nucleation layer 154 and a buffer layer 156 formed ofa variety of different materials as described above.

FIG. 7 also illustrates the structure after undergoing an epitaxialgrowth process to form a first heterostructure 157 over the basestructure 150. The first heterostructure 157 is formed of an AlGaN layer162 overlying a GaN layer 158. The GaN layer 158 is deposited over thebase structure 150 and the AlGaN layer 162 is deposited over the GaNlayer 158. Any suitable technique for depositing each layer can beemployed such as metal organic chemical vapor deposition (MOVCD),molecular beam epitaxy (MBE) or other suitable deposition techniques.The AlGaN layer 162 is doped with a dopant to induce carriers andreplacing the effect that would have occurred but for the balancingeffect of multiple symmetric heterostructures. The dopant is selected toprovide the AlGaN layer 162 with a first doping concentration based onthe resultant thicker width of the first heterostructure relative tooverlying heterostructures after a subsequent etch to form superlatticeheterostructure ridges. The resultant structure is illustrated in FIG.8.

FIG. 9 illustrates the structure of FIG. 8 after a secondheterostructure 163 is formed over the first heterostructure 157. Thesecond heterostructure 163 is formed of an AlGaN layer 166 overlying aGaN layer 164. However, the GaN layer 164 is of a greater thickness thanthe GaN layer 158 of the first heterostructure 157 to facilitate effectof allowing the fringing electric fields to penetrate into the interiorof the ridges of the middle channel formed by the second heterostructure163, thus aiding in pinch-off and reducing the overall pinch-off voltageassociated with the second heterostructure 163. The AlGaN layer 166 isdoped with a dopant to induce carriers and replace the effect that wouldhave occurred but for the balancing effect of multiple symmetricheterostructures. The dopant is selected to provide the AlGaN layer 166with a second doping concentration that is greater than the first dopingconcentration based on the resultant shorter width of the secondheterostructure 163 relative to the underlying first heterostructure 157after a subsequent etch to form superlattice ridges. The resultantstructure is illustrated in FIG. 10. The dots of the doped AlGaN layer168 of the second heterostructure 163 are increased in density relativeto the dots in the AlGaN layer 162 of the first heterostructure 157 toshow the increase in doping concentration.

It should be noted that for illustrative purposes, the bottom GaN isshown as being thinner than the thicker GaN. However, in practice, aback barrier structure of about 50 nm GaN is below the bottom AlGaNbarrier followed by a 4% AlGaN back barrier underneath that to preventcurrent leakage under the gated ridges.

FIG. 10 illustrates the structure of FIG. 9 after a thirdheterostructure 169 is deposited over the second heterostructure 163.The third heterostructure 169 is formed of an AlGaN layer 172 overlyinga GaN layer 170. However, the GaN layer 170 of the third heterostructure169 has relatively the same thickness of the GaN layer 158 of the firstheterostructure 157 since fringing fields are not blocked on a top ofthe third heterostructure 169 and a bottom of the first heterostructure157. The AlGaN layer 172 is doped with a dopant to induce additionallycarriers and replace the effect that would have occurred but for thebalancing effect of multiple symmetric heterostructures. The dopant isselected to provide the AlGaN layer 172 with a third dopingconcentration that is greater than the second doping concentration andgreater than the first doping concentration based on the resultantshorter width of the third heterostructure 169 relative to theunderlying second heterostructure 163 and the underlying firstheterostructure 157. The dots of the doped AlGaN layer 172 of the thirdheterostructure 169 are increased in density relative to the dots in theAlGaN layer 168 of the second heterostructure 163 to show the increasein doping concentration.

As illustrated in the cross-sectional view of FIG. 11, an etch mask 176is formed over the superlattice structure formed from the first, secondand third heterostructures 157, 163, and 169, respectively. The etchmask 176 can be formed by depositing, patterning and developing aphotoresist material layer over the superlattice structure, depositing ahard mask material, such as for example, a thick layer of metal (e.g.,nickel) (about 300 Å) or the like in the patterned openings, and thenremoving the photoresist material layer to leave a resultant hard mask.The hard mask 176 provides covered regions that protect underylyingareas of the superlattice structure, where the multi-ridgemulti-channels will reside, and also covered regions that protect thesource and drain regions (not shown). The mask 176 also includesopenings 179 over the superlattice structure exposing a top layer ofAlGaN 172 for forming underlying castellated non-channel openings 181(see FIG. 12). Alternatively, a hard mask material such as an insulatormaterial (e.g., silicon oxide (SiO₂), silicon nitride (SiN), etc.) canbe deposited over the superlattice structure followed by a depositingand patterning of photoresist material with a subsequent etching of theinsulator material and stripping of the photoresist material to form theresultant mask 176.

FIG. 11 illustrates performing of an etch 230 (e.g., anisotropicreactive ion etching (RIE)) to remove portion of the superlatticestructure under the openings 179 in the etch mask 176 for formingextended openings 181 (FIG. 12) in the superlattice structure down tothe base structure 150. The etch step 230 can be a dry etch that employsan etchant which selectively etches the material of the supperlaticestructure at a faster rate than the underlying base structure 150 andthe overlying etch mask 176. For example, the supperlattice structuremay be anisotropically etched with a plasma gas(es), herein carbontetrafloride (CF₄) containing fluorine ions, in a commercially availableetcher, such as a parallel plate RIE apparatus or, alternatively, anelectron cyclotron resonance (ECR) plasma reactor to replicate the maskpattern of the etch mask 176 to thereby remove portion of thesuperlattice structure down to the base structure 150 leaving aplurality of multichannel ridges 180. Alternatively, the etching process230 can include etching the channel region by an inductively coupledplasma (ICP) etcher to form “castellations” in the form of a pluralityof multichannel ridges 180 and non-channel openings 181. The etch mask176 is stripped to provide the resultant structure of FIG. 12. Aspreviously stated, limits in fabrication cause these ridges 180 to beetched with sloped sidewalls instead of straight sidewalls.

FIG. 13 provides a cross-sectional view of the structure of FIG. 12after undergoing a gate contact fill deposition along the longitudinalopening with a conductive material to form three sided gate contact 182that wraps around and substantially surrounds three sides of each of theplurality of multichannel ridges 180. Optionally, a dielectric layer canbe deposited that wraps around and surrounds the three sides of each ofthe plurality of multichannel ridges 180 along at least a portion oftheir lengths prior to the gate contact fill to facilitate the reductionin leakage current by the resultant device structures. Alternatively,the gate contact material can be deposited over the entire structure andetched away everywhere but over the longitudinal opening. The gatecontact fill deposition can be an atomic layer deposition or otherdeposition technique conducive to depositing into trench openings andundercut openings. The gate contact 182 can be made of a conventionalcontact material comprising layers of nickel and gold, as known in theart. Other gate contacts compositions will be apparent to those skilledin the art. Subsequently or concurrently drain and source contacts canbe formed, and an overlying passivation layer deposited over the finalstructure to form the final integrated SLFET device.

What have been described above are examples of the invention. It is, ofcourse, not possible to describe every conceivable combination ofcomponents or methodologies for purposes of describing the invention,but one of ordinary skill in the art will recognize that many furthercombinations and permutations of the invention are possible.Accordingly, the invention is intended to embrace all such alterations,modifications, and variations that fall within the scope of thisapplication, including the appended claims.

What is claimed is:
 1. A method of forming a transistor device, themethod comprising: forming a superlattice structure comprising aplurality of heterostructures over a base structure by sequentiallydepositing each layer of a plurality heterostructures over the basestructure with one layer of each heterostructure being doped; etchingaway openings in the superlattice structure over a channel region toform a castellated region in the channel region of alternatingmultichannel ridges with edges and non-channel openings, wherein aparameter of at least one of corresponding parallel heterostructures ofeach of the multichannel ridges is varied; and performing a gate contactfill process to form a gate contact that wraps around and substantiallysurrounds the top and sides of each the plurality of multichannel ridgesalong at least a portion of its depth and connects each one of thealternating multichannel bridges to one another through the non-channelopenings.
 2. The method of claim 1, wherein the parameter is a dopantconcentration.
 3. The method of claim 1, wherein the parameter is adopant concentration for each heterostructure, such that longer widthchannels associated with a given heterostructure are doped with lessdopant concentration than shorter width channels associated with a givenheterostructure for each of the plurality of heterostructures tosubstantially equalize the pinch-off voltage of each channel of theSLCFET during operation.
 4. The method of claim 1, wherein the parameteris a thickness.
 5. The method of claim 1, wherein the parameter is athickness, such that each inner heterostructure is formed with a greaterthickness than a thickness of at least one of a top heterostructure anda bottom heterostructure for each of the multichannel ridges.
 6. Themethod of claim 1, wherein the parameter is a thickness, such that everyother inner heterostructure is formed with a greater thickness than athickness of at least one of a top heterostructure and a bottomheterostructure for each of the multichannel ridges.